PSI - Issue 64

Marco Carlo Rampini et al. / Procedia Structural Integrity 64 (2024) 2141–2148 Author name / Structural Integrity Procedia 00 (2019) 000–000

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surfaces of the core bars (Ø30 and Ø16 mm ones). Unfortunately, at the present time, no documents related to the previous maintenance intervention are available. It can be only stated that this action was probably carried out before 2005, because from a technical report on structural activities ( i.e. sampling and indirect resistance evaluation by sclerometer and ultrasonic measures, and identification of the vibrational response of the deck) carried out in a period of four months across 2005 and 2006, the same observations previously carried out on the characteristics of the tie rods were already present. As visible from Figure 1b-c, after their removal the RC hangers were stored on the deck and then transferred to the laboratory to proceed with the diagnosis presented in the next sections. A total of 10 tie rods were collected, ranging between 4.0 and 7.1 m of length, and identified as S (south side) and with a progressive number from 6 to 15 (Figure 2). Due to the removal process, only the middle part of the tie rods remained integer. At the present preliminary state of work, the investigation was performed on three RC members only. The diagnostic campaign was performed in two phases. First of all, electrochemical measurements were carried out, to assess the electrical resistivity of the cementitious mortar and the corrosion potential of the steel bars. These measurements were performed in order to obtain a spatial mapping of the electrochemical parameters along the length of each tie rod. According to the results obtained, carbonation depth was investigated in some selected points, where electrochemical measurements highlighted the most critical conditions. Half of the external metallic cover was removed (leaving in place the other half, in contact with the ground), just prior to execute the electrochemical measurements, to limit the alteration of moisture content of the cementitious composite layer. While performing electrochemical measurements, a visual inspection and identification of possible defects on the external surface of the cementitious layer were also carried out. More details on the diagnostic techniques are summarized below. 1) Electrical resistivity: Two-point probe was employed, connected to a conductivity meter. Surface electrical conductivity was measured and subsequently transformed into electrical resistivity, considering the cell constant of the probe. Measurements were performed each 10 cm along the length of the tie rod, and considering for each position three equidistant points on the external semicircular surface, i.e. A, B and C (see Figures 5, 6, and 8). 2) Corrosion potential: Corrosion potential of the bars was measured through a voltmeter, with respect to an external copper/copper sulphate (CSE) reference electrode. The measurements were performed each 10 cm along the length of each tie rod, only in the B position (top face of the specimen in the investigated configuration). The electrical contact with the steel bars was obtained on one of the two ends of the tie rod, by using an electrical crocodile clip directly connected to one of the core rebar. To improve the electrical connection, mechanical cleaning was performed to remove from the bar surface the protective paint and any corrosion products. The electrical contact among the several steel bars was checked, and always verified for at least the core bars of the tie rod, which are the most relevant from a structural point of view as already discussed. The procedure and the interpretation of the results were carried out in accordance with Elsener et al. (2003). a b c 3. Diagnostic campaign 3.1. Diagnostic techniques

Fig. 4. Phases for the concrete core sampling to perform carbonation assessment.

3) Carbonation depth: Carbonation was investigated on extracted samples, considering the penetration depth from the outer surface in as radial a direction as possible. The phases of the concrete extraction are depicted in Figure 4, and particular attention was kept in order to prevent the damage of the internal bars. Right after collection, a solution of phenolphthalein was applied on the vertical walls